Magnetic gradiometer based on a high-transition temperature superconducting quantum interference device for improved sensitivity of a biosensor

We describe a gradiometer based on a high-transition temperature superconducting quantum interference device (SQUID) that improves the sensitivity of a SQUID-based biosensor. The first-derivative gradiometer, fabricated from a single layer of YBa2Cu3O7−x, has a baseline of 480 μm and a balance against uniform fields of 1 part in 150. Used in our SQUID “microscope,” it reduces the response to parasitic magnetic fields generated by the measurement process to the level of the SQUID noise. The gradiometer-based microscope is two orders of magnitude more sensitive to superparamagnetic nanoparticles bound to biological targets than our earlier magnetometer-based microscope.